<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            width: 100%;
            height: 600px;
            border: 1px solid green;
        }

        .box div {
            float: left;
        }

        .bl {
            width: 100px;
            height: 600px;
            background: lightcoral;
        }

        /*  calc 可以进行css的计算，用于布局左侧宽度固定，右侧充满其余的屏幕  */
        .br {
            width: calc(100% - 100px);
            height: 600px;
            background: lightseagreen;
        }
    </style>
</head>

<body>
    <div class="box">
        <div class="bl"></div>
        <div class="br"></div>
    </div>
</body>

</html>